How to separate iPhone from iPad on different accounts?

You cannot create a new Apple Account (previously known as an AppleID) using an email address that already identifies another Apple Account. You must use a different email address.

An Apple Account is intended to be used with all devices owned by a single user - as this allows you to share purchased Apps and subscribed Apple Services between all your devices. What, specifically, is your goal in using a completely different Apple Account for your new iPad?

If you really must use a different Apple Account, consider using an account that can be created (for free) from Google or Microsoft.

On your iPad and iPhone, you simply need to modify settings related to the App Store:

Settings > Apps > App Store > [Automatic Downloads]

• App Downloads - set to OFF
• App Updates - set to OFF
• In-App Content - set to OFF

Just set the preferences in each device not to automatically download new purchases:

Settings -> App Store

Regards.